Investigations in non-human primates have demonstrated a marked circadian pattern of change in cerebrospinal fluid cortisol concentrations. Peak to trough differences in CSF cortisol are greater than those seen in plasma cortisol concentrations. Other neuroendocrine substances (e.g., prolactin and beta-endorphin) as well as amines and amine metabolites are being studied in relation to neurotransmitter-related drug effects. Measurement of these same substances in cat and rodent cerebrospinal fluid and brain supplement the studies done in monkeys and man.